The use of acoustic (e.g., audible and/or ultrasonic) measurement systems in prior art downhole applications, such as logging while drilling (LWD), measurement while drilling (MWD), and wireline logging applications, is well known. Such acoustic measurement systems are utilized in a variety of downhole applications including, for example, borehole caliper measurements, measurement of drilling fluid properties, and the determination of various physical properties of a formation. In one application, acoustic waveforms may be generated at one or more transmitters deployed in the borehole. The acoustic responses may then be received at an array of longitudinally spaced apart receivers deployed in the borehole. Acoustic logging in this manner provides an important set of borehole data and is commonly used in both LWD and wireline applications to determine compressional and shear wave velocities (also referred to as slowness) of a formation.
It will be appreciated that the terms slowness and velocity are often used interchangeably in the art. They will likewise be used interchangeably herein with the understanding that they are inversely related to one another and that the measurement of either may be converted to the other by simple and known mathematical calculations. Additionally, as used in the art, there is not always a clear distinction between the terms LWD and MWD. Generally speaking MWD typically refers to measurements taken for the purpose of drilling the well (e.g., navigation) whereas LWD typically refers to measurements taken for the purpose of analysis of the formation and surrounding borehole conditions. Nevertheless, these terms are herein used synonymously and interchangeably.
In the analysis of acoustic logging measurements, the received acoustic waveforms are typically coherence processed to obtain a time-slowness plot. In a time-slowness plot, also referred to as a slowness-time-coherence (STC) plot or a semblance plot, a set of several signals from the array of acoustic receivers is processed with the incorporation of separate time shifts for each received signal. The separate time shifts are based on a slowness value assumed for the purpose of processing the waveforms. The processing provides a result, known as coherence, which can signify the presence of a discernable signal received by the separate receivers. In this manner compressional and shear wave arrivals can be discerned in the received waveforms, leading to determinations of their velocities. The determined compressional and shear wave velocities are related to compressive and shear strengths of the surrounding formation, and thus provide useful information about the formation.
In acoustically slow formations, in which the velocity of formation shear waves is less than the speed of sound in the drilling fluid (mud), shear wave slowness determination is known to be complicated by poor transmission of shear wave energy across the boundary between the formation and the borehole. Various techniques have been developed for determining shear wave slowness in acoustically slow formations. These techniques commonly involve exciting relatively pure mode borehole guided waves (e.g., monopole, dipole, or quadrupole mode waves). The shear wave slowness is then estimated from the guided wave velocity of these pure modes.
Unfortunately, guided wave propagation tends to be highly dispersive in LWD applications. Although STC analysis is widely used, dispersive effects in the received waveforms can reduce the reliability of the STC analysis. By dispersive it is meant that the guided wave slowness depends on the frequency at which the wave propagates. Many factors contribute to the amount of slowness dispersion. These factors include, for example, tool body properties, eccentricity, borehole diameter, formation shear slowness and compressional slowness, and mud density and slowness. In order to obtain a suitably accurate shear slowness value, processing is required that relies upon values for these factors. This processing is commonly referred to in the art as “dispersion correction”. In many applications, values for each of these other factors are not accurately known, which can in turn lead to errors in the shear slowness estimate.
Another drawback with the aforementioned techniques is that logging while drilling tools configured for transmitting and/or receiving relatively pure acoustic modes require highly complex transmitter and/or receiver configurations, which tend to be expensive. For example, transmitters configured to produce a pure acoustic mode typically include numerous (e.g., four, eight, or even more) distinct transducer elements deployed about the circumference of the tool body. In order to produce a pure mode and to suppress other modes, highly precise phasing (timing) of the various transducers is typically further required. The difficulty in generating such acoustic signals is also known to be further exacerbated by tool eccentricity in the borehole (e.g., in highly deviated wells in which the tool typically lies on the low side of the borehole). Moreover, the use of such complex transmitters and receivers in severe downhole conditions including extreme temperatures and pressures and severe mechanical shocks and vibrations tends to reduce tool reliability.
Therefore, there exists a need for an improved logging while drilling tool. In particular, there is a need for an improved logging while drilling tool suitable for determining shear wave slowness in acoustically slow formations.